1. Field of the Invention
The present invention relates to a substrate surface inspection method and inspection apparatus, and particularly to a substrate surface inspection method and inspection apparatus for detecting a defect on a substrate.
2. Description of the Related Arts
Conventionally, a foreign material inspection method that uses light is known. For example, a conventional foreign material inspection method comprises: irradiating a photomask or other sample with light; receiving light reflected from the sample and light transmitted through the sample to image the sample; and detecting a foreign material by means of a foreign material detection threshold. Such a method is disclosed, for example, in Japanese Patent Laid-Open Application No. 2008-96296.
In the above-described foreign material inspection method, however, the detection limit for a foreign material on a reticle is deemed to be about 50 nm since the method uses light. In recent years, line widths on a reticle have become narrower as pattern sizes of semiconductors have become narrower, so even a small foreign material would be a serious defect. Conventional inspection apparatuses of a light type would have a problem of not being able to detect a foreign material of 50 nm or less in size.
In addition, it would be difficult to distinguish a foreign material on a pattern on a reticle from part of the pattern shape, which would also be a problem. FIGS. 12A to 12C are schematic views of a conventional cell comparison inspection.
FIG. 12A is a schematic view showing one example of a die to be inspected. In FIG. 12A, a pattern 220 including cell areas having a regular pitch is provided on a reticle surface 210. Suppose that the reticle surface 210 is irradiated with an electron beam to be imaged and the presence or absence of a foreign material is inspected for by comparing cells. The cell comparison inspection comprises: acquiring images at a preset cell pitch; comparing those images; and determining from a difference between the images the presence or absence of a defect such as a foreign material 230.
FIG. 12B is a schematic view showing a difference image acquired by the cell inspection. As shown in FIG. 12B here, sections other than a repetitive pattern, i.e. random pattern sections or the like, exist on the image as patterns other than a foreign material (background). Such sections other than a repetitive pattern are all detected as pseudo defects (pseudo foreign materials, in this case).
FIG. 12C shows a principle of the defect detection using the cell comparison. The cell comparison inspection presupposes that patterns repeated at a pitch exist in the inspection area. The minimum unit of the repeated patterns is set as the cell pitch. The defect inspection determines a difference between a signal of a cell on which attention is focused and a signal of the preceding cell. If the difference is zero, one pattern is repeated. If the difference is not zero, there is a shape that is not the minimum unit of the repeated patterns. This shape that is not the repetitive pattern is detected as a defect signal 235.
In a section where a pattern changes or the like, however, there is no repeated-at-a-pitch pattern to be presupposed but there are irregular patterns. The cell pitch cannot be set for such a section, and therefore the difference value of the cell comparison becomes other than zero in a section where a pattern changes or the like. As a result, every such section would be detected as a pseudo defect signal 225, which is the problem.
A die comparison is known as an inspection method for preventing such a problem. FIGS. 13A to 13C are schematic views showing a conventionally used die comparison inspection. FIG. 13A is a schematic view of a reference die, and FIG. 13B is a schematic view of a die to be inspected. FIG. 13C shows a difference image between the reference die and the die to be inspected.
Generally, the die comparison inspection is carried out when a pattern 220 other than a repetitive pattern is inspected for a pattern defect. Since the die size is generally much larger than a cell pitch, the travel of the stage is correspondingly long. For this reason, the positioning accuracy, speed accuracy, rotation angle control accuracy, and the like of the stage are required to be high, that is, the mechanism, control, and the like of the stage are required to be highly precise. The apparatus would therefore be expensive. The image processing algorithm for the image comparison would be complicated, the image processing would correspondingly require time, and the determination of the presence or absence of the foreign material 230 would require a great deal of time.
In contrast to such a die comparison inspection, the cell comparison inspection can accurately carry out an inspection with less cost of the stage mechanism, control algorithm, and the like. The cell comparison inspection also has an advantage of being able to make the inspection time shorter than the die comparison inspection. However, as described above, it is required to solve the problem in which the background interferes when the cell comparison inspection is used to inspect for a foreign material.